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Some stellar general purpose utils.
There is a newer version: 0.7
/**
 * Copyright (C) Posten Norge AS
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *         http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package no.digipost.collection;

import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Stream;

import static java.util.Collections.unmodifiableSet;
import static java.util.stream.Collector.Characteristics.IDENTITY_FINISH;
import static java.util.stream.Collectors.toSet;

/**
 * A {@link Collector} which offers some means for adapting it into new Collectors,
 * in particular the ability to {@link #andThen(Function) map the result}.
 */
public final class AdaptableCollector implements Collector {

    public static  AdaptableCollector of(Supplier supplier,
                                                          BiConsumer accumulator,
                                                          BinaryOperator combiner,
                                                          Function finisher,
                                                          Set characteristics) {
        return new AdaptableCollector<>(supplier, accumulator, combiner, finisher, characteristics.stream());
    }

    public static  AdaptableCollector of(Supplier supplier,
                                                           BiConsumer accumulator,
                                                           BinaryOperator combiner,
                                                           Function finisher,
                                                           Characteristics ... characteristics) {
        return new AdaptableCollector<>(supplier, accumulator, combiner, finisher, Stream.of(characteristics));
    }





    private final Supplier supplier;
    private final BiConsumer accumulator;
    private final BinaryOperator combiner;
    private final Function finisher;
    private final Set characteristics;

    private AdaptableCollector(Supplier supplier,
                               BiConsumer accumulator,
                               BinaryOperator combiner,
                               Function finisher,
                               Stream characteristics) {
        this.supplier = supplier;
        this.accumulator = accumulator;
        this.combiner = combiner;
        this.finisher = finisher;
        this.characteristics = unmodifiableSet(characteristics.collect(toSet()));
    }

    /**
     * Create a new {@code Collector} which maps the resulting container to something else
     * using the given {@code resultMapper} {@link Function}.
     *
     * @param  The new result type after applying the mapper functions.
     * @param resultMapper the function used to map the collector's result
     *
     * @return a new Collector
     */
    public  AdaptableCollector andThen(Function resultMapper) {
        return new AdaptableCollector<>(supplier, accumulator, combiner, finisher.andThen(resultMapper), characteristics.stream().filter(c -> c != IDENTITY_FINISH));
    }

    @Override
    public Supplier supplier() {
        return supplier;
    }

    @Override
    public BiConsumer accumulator() {
        return accumulator;
    }

    @Override
    public BinaryOperator combiner() {
        return combiner;
    }

    @Override
    public Function finisher() {
        return finisher;
    }

    @Override
    public Set characteristics() {
        return characteristics;
    }

}